92 Williamson. — Some Experiments on the Action of 
in contact with the plate, and secure it in place by means of a rubber band. 
It was enclosed in black paper and then placed in a cardboard box. It was 
found necessary to take further precautions against any light filtering 
through by placing the boxes in a tin, blackened on the inner surface and 
possessing a tight-fitting lid. After numerous experiments it was decided 
that, for the seasoned timber used, process plates were the most satisfactory, 
so Ilford process plates were used throughout. The most reliable developers 
were pyro soda made up without any bromide and Imperial hydrokinone. 
I am inclined to think the latter gave the best results, and it was the one 
used in the most critical work. 
The optimum temperature and time of exposure were ascertained by 
preliminary experiments on Oak. The first plates were failures ; in these 
cases air-dried oak was used and it did not give more than a faint negative. 
A plate was then prepared with two pieces of oak, one of which had been 
left in a moist atmosphere at 56° C. for forty-two hours and the other one 
was air-dry. The former gave a sharper image with greater density. It 
appeared that a certain amount of moisture was necessary to produce a good 
negative, so wherever air-dried material was used it was first exposed to 
moist air for times varying from 1 to 3 days. The factors of time and 
temperature of exposure interact. Generally speaking, the higher the 
temperature the shorter the exposure required, e. g. a good negative was 
obtained with oak in 72 hours at 30° C. ; the same material gave as good 
a result when the exposure was 24 hours at 40° C. At 50° C. an exposure 
of 5 hours was sufficient, but the film was slightly injured. On the whole 
an exposure of 24 hours at 40° C. was found satisfactory. 
Oak leaves an exact picture of itself on a photographic plate in the 
dark. The spring wood is inactive, giving a light band, and the autumn 
wood is active, giving a dark band showing the tracheides and fibres and 
parenchyma ; the medullary rays are also clearly visible as dark lines. 
Fig. 1 shows a print of such an image, giving the positive. For convenience 
it is proposed to call the wood that acts on a photographic plate, giving 
a dark band, positive, and the wood that fails to act, so producing a light 
band, negative. In the oak, then, the spring wood is negative and the 
autumn wood is positive. In order to be quite certain of the interpretation 
of these positive and negative bands, two thin paper indicators were gummed 
on to the wood, one on a ring of spring wood and one on a ring of autumn 
wood. The wood does not act through the paper, so an active ring of wood 
produces a dark band on the film interrupted by^a white line in the posi- 
tion of the paper. A comparison of the positions of the indicators on the 
wood and the negative soon determines whether it is the spring or autumn 
wood that is positive. Images taken without this precaution are apt to be 
misleading. 
